In a large saucepan, heat 2 tablespoons of olive oil over medium heat. Add 1 finely diced onion and sauté until soft and translucent, about 5 minutes. Stir in 3 minced garlic cloves and a pinch of crushed red pepper flakes, cooking for an additional minute. Pour in one 28-ounce can of crushed tomatoes, add salt, black pepper, granulated sugar, and fresh basil, then let simmer for 45 minutes, stirring occasionally until thickened.

In a mixing bowl, combine 15 ounces of ricotta cheese, 1 cup of grated Parmesan, 1 cup of shredded mozzarella, and 1 beaten egg. Add freshly chopped parsley and basil, alongside a sprinkle of salt and black pepper. Mix until the cheese filling is smooth and creamy. Set it aside, ensuring the filling is ready for the Classic Stuffed Shells assembly.

Bring a large pot of salted water to a boil. Carefully add the jumbo shells and cook for only 4 minutes, ensuring they remain firm. Drain the shells and rinse them under cool running water to stop the cooking process. This step helps preserve their shape for stuffing later.

Preheat your oven to 375°F (190°C). Spread a thin layer of marinara sauce along the bottom of a baking dish to prevent sticking. Take each shell and use a heaping tablespoon of the cheese filling to stuff it generously. Place the stuffed shells seam-side up into the prepared dish, making sure they fit snugly together for optimal baking.

Pour the remaining marinara sauce over the stuffed shells, covering them evenly. Sprinkle the last of the mozzarella cheese atop the sauce, allowing for a gooey, cheesy finish. Bake in the preheated oven for 35 minutes, until the cheese is melted and bubbly, creating a golden brown crust.

Once baked, remove the Classic Stuffed Shells from the oven and allow them to rest for about 5 minutes. Garnish with fresh parsley and additional grated Parmesan for color and flavor. Slice, serve, and relish the hearty goodness with your family.